Protective coatings offer the prospect of minimizing material degradation under the severe operating environments commonly encountered in the aircraft, petroleum, chemical and synfuels industries. Metallic coatings based on Cr, Al and Si, either singly or in combination, have been in use for several decades to enhance environmental resistance of materials to high temperature corrosion, including high temperature oxidation and hot corrosion, particle erosion, ereosion-corrosion, wear and thermal degradation. Aluminum diffusion coatings (or "aluminide" coatings) represent by far the most widely used coatings. Much of the early development of these coatings was in the aircraft industry for jet engine applications. Demands of high temperature strength of substrate materials for these applications necessitated the use of Ni-base and Co-base superalloys. In contrast to the superalloys, Al diffusion coating technology for ferrous systems, such as the heat-resistant austenitic stainless steels, is lacking understanding and optimization. Heat-resistant stainless steels are widely used in petroleum, chemical, nuclear and other applications due to their excellent intermediate and high temperature strength and room temperature fabricability. Aluminizing of these stainless steels has thus received revived impetus of late as a means of improving the high temperature corrosion resistance of materials likely to be considered in many energy related applications.
Alonizing and other state-of-the-art aluminizing processes as conventionnaly carried out for surface modification of stainless and other steels invariably result in the formation of a multi-layer coating consisting of a major, continuous outer layer phase on intermetallic compounds [aluminides, (Fe,Ni).sub.2 Al.sub.3 or (Ni,Fe)Al or mixtures thereof] and as well a sub-layer of interdiffused zone made up of a continuous metal matrix, see FIG. 1 (A) labeled "Commercial Processing".
It has been found that the extremely brittle and crack-prone outer aluminide layer of the stateof-the-art aluminum diffusion coatings is not durable under field exposure conditions and besides does not provide the desired corrosion resistance for those applications involving intermediate to low temperature corrosive environments, such as experienced in coal gasification. In FIG. 2, it is seen that the outer aluminizing layer is cracked and non-durable in the three examples shown. On the other hand, the sub-layer has been found to have a high hardness in relation to the substrate, substantially higher toughness, durability and chemical resistance properties than those of the outer aluminide layer. For example, in alkali catalyzed gasification processes alkali was found to penetrate through cracks in the exterior aluminide layer on a commercially aluminized type 310SS, but not through the interdiffused sub-layer, see, e.g., Bangaru and Krutenat, J. Vac. Sci. Technol., B2(4), Oct.-Dec, 1984. Critical examination of the interdiffused sublayer showed that it consists of a composite microstructure containing a dispersion of aluminide particles which are coherently bonded to the chromium enriched ferrite matrix, see FIG. 3. The dark particles in these micrographs are the alumind particles dispersed in a continuous ferrite matrix. This and the preferential portioning of steel's Si into the ferrite matrix provide the interdiffusion sub-layer a higher hardness than that of substrate austenite and a greater toughness than that of the exterior aluminide layer.
For low to intermediate temperature aggressive environments involving relatively high PS.sub.2 and low PO.sub.2 (i.e., reducing-sulfidizing), such as those encountered in many energy conversion processes, it has been shown that a high Cr level in the coating besides Al is beneficial. Thus, currently, duplex Cr-Al rich coatings are specified for these applications. Most widely used commercial duplex Cr-Al coatings are produced by a two-step process wherein a high temperature Cr-diffusion coating is formed first which is subsequently aluminized to form the duplex coating. One such company providing coatings is Alloy Surfaces Co., Wilmington, Delaware. These commercial duplex coatings suffer from the same disadvantages as those of simple aluminizing, i.e., formation of an outer brittle aluminide continuous layer.
Besides the environmental and mechanical debits associated with the formation of the exterior aluminide layer, it has also further implications on the ability to co-diffuse elements. The diffusion of Cr, for example, is extremely slow through the aluminide, thereby eliminating the possibility of co-diffusion of Cr and Al into the steel under normal coating conditions wherein the kinetics of aluminide formation are favored.
One objective of this invention is to produce on Type 304, 316, 310 austenitic stainless steels and others of similar composition a single layer coating consisting only of the interdiffused region, i.e.g, without the continuous exterior aluminide, by a novel diffusion aluminizing process. This allows the formation of a thick coating layer, see FIG. 1(B) labeled "New Conrolled Activity Processing". The essential requirements for this process to succeed is to maintain the activity of Al in the source equal to or below a level which precludes the formation of a continuous outer aluminide layer.
A second objective of the present invention is to co-diffuse two or more elements simultaneously by taking advantage of the absence of diffusion inhibiting exterior aluminide layer. Specifically, single-step Cr-Al rich duplex diffusion coating is produced on substrates of interest made up only of the interdiffusion layer.
A key objective of this invention is to create by diffusion alloying coatings of significant thickness (up to .about.20 mils or .about.500 .mu.m) with sufficient Al such as to be "Al.sub.2 O.sub.3 formers" during oxidation at high temperature, and sufficient Cr for "hot corrosion" resistance, i.e., resistance to sulfur attack.